1. Field of the Invention
The present invention relates to a protection apparatus employing a thermal fuse, including a protection circuit to extend the area of use of the thermal fuse to equipment directed to high voltage and high current. Particularly, the present invention relates to a protection apparatus that can avoid an abnormal event immediately after activation of a thermal fuse by using an electrosensitive fuse with the thermal fuse.
2. Description of the Background Art
A thermal fuse is a protection component to properly sense abnormal overheating at the electric apparatus and quickly cut off the circuit. A thermal fuse is employed in various home electrical products, portable apparatuses, communication equipment, business machines, in-car devices, air conditioners, AC adapters, chargers, batteries, and electronic components. In general, thermal fuses are mainly classified into two types depending upon the thermosensitive material employed. Specifically, there are known a fusible alloy type thermal fuse using conductive low-melting fusible alloy for the thermosensitive material, and a thermosensitive pellet type thermal fuse employing a non-conductive thermosensitive substance. Both are activated in response to sensing abnormal temperature rise at the electrical apparatus to which it is attached to cut off the current to the electric apparatus. Both function to protect electrical equipment by switching the conductive state of the current-carrying path, and are also referred to as “non-reset thermal switches”. In other words, they are protection means for electric products achieving a cut-off state by reversing the conductive state in the initial ordinary temperature state at a predetermined operating temperature. The operating temperature for activation depends on the thermal sensitive material employed. In general, the operating temperature is 60° C. to 250° C. A wide selection of general-purpose protection components that function with the rated current in the range of 0.5 A to 15 A is commercially available.
For example, as disclosed in Japanese Patent Laying-Open Nos. 2005-158681 and 2003-317589, a thermosensitive pellet type thermal fuse that has the characteristics of low internal resistance and high breaking current allows an operating temperature to be set arbitrarily over a wide range by employing a thermosensitive pellet formed of thermoplastic resin. A fusible alloy type thermal fuse with a low cut-off current, hermetically sealed in an insulative case to detect the temperature, has flux attached to low-melting fusible alloy to achieve cut off by rendering the fusible alloy globular when fused, and has a relatively low operating temperature of 60° C.-230° C. As disclosed in Japanese Patent Laying-Open Nos. 06-243767 and 04-282523, the flux attached on the surface of the low-melting fusible alloy serves to prevent disturbance of an oxide film and break the electrical connection between electrodes through the fusible alloy that melts at a predetermined temperature and rendered globular by the surface tension, when the low melting fusible alloy is fused at the melting temperature.
Current fuses include various types such as the glass-tube type, the time-lag type that operates with delay, the high withstand voltage and high current type, and the like. In general, the regulation calls for activation within 2 minutes with respect to overcurrent of 200% the rated current. There is also known a fuse that is activated at an elapse of at least one minute of the conducting duration even if the current is below 2 times the rated current. There is also known a resistance fuse that is locally made thin to be blown out by the Joule heat caused by the resistance. In a circuit that uses such a current fuse that interrupts the circuit in response to sensing current, a hazardous condition may be induced by overheating due to generation of Joule heat by the load per se and/or rise of the ambient temperature. To avoid this critical condition, a thermal fuse is used together to cut off the circuit safely to eliminate an overheating state.
A composite structure using a thermal fuse and a current fuse together is also known. The fuse elements are arranged in series connection on the same substrate in an insulative package, including an intermediate electrode. This type of composite fuse is known, as disclosed in Japanese Patent Laying-Open No. 2003-297206, for example. The composite fuse has the tips of a pair of lead conductors secured to a resin base film with an intermediate electrode between the conductor tips, wherein a thermal fuse element and a current fuse element are connected at one side and the other side, respectively. Further, Japanese Patent Laying-Open No. 2000-123694 discloses a composite fuse with a thermal fuse element that is blown by sensing heat generated by a current fuse element. Japanese Patent Laying-Open No. 2000-133102 discloses a composite structure of a current fuse and a thermal fuse, each fuse element connected between an intermediate electrode and each tip of a pair of lead conductors, using a lead frame constituting an outer frame.